CVE-2021-47013

In the Linux kernel, the following vulnerability has been resolved:

net:emac/emac-mac: Fix a use after free in emac_mac_tx_buf_send

In emac_mac_tx_buf_send, it calls emac_tx_fill_tpd(..,skb,..). If some error happens in emac_tx_fill_tpd(), the skb will be freed via dev_kfree_skb(skb) in error branch of emac_tx_fill_tpd(). But the freed skb is still used via skb->len by netdev_sent_queue(,skb->len).

As i observed that emac_tx_fill_tpd() havent modified the value of skb->len, thus my patch assigns skb->len to len before the possible free and use len instead of skb->len later.

Weakness

Referencing memory after it has been freed can cause a program to crash, use unexpected values, or execute code.

Affected Software

Extended Description

The use of previously-freed memory can have any number of adverse consequences, ranging from the corruption of valid data to the execution of arbitrary code, depending on the instantiation and timing of the flaw. The simplest way data corruption may occur involves the system’s reuse of the freed memory. Use-after-free errors have two common and sometimes overlapping causes:

In this scenario, the memory in question is allocated to another pointer validly at some point after it has been freed. The original pointer to the freed memory is used again and points to somewhere within the new allocation. As the data is changed, it corrupts the validly used memory; this induces undefined behavior in the process. If the newly allocated data happens to hold a class, in C++ for example, various function pointers may be scattered within the heap data. If one of these function pointers is overwritten with an address to valid shellcode, execution of arbitrary code can be achieved.

Potential Mitigations

References

• https://git.kernel.org/stable/c/16d8c44be52e3650917736d45f5904384a9da834
• https://git.kernel.org/stable/c/55fcdd1258faaecca74b91b88cc0921f9edd775d
• https://git.kernel.org/stable/c/6d72e7c767acbbdd44ebc7d89c6690b405b32b57
• https://git.kernel.org/stable/c/8c06f34785068b87e2b560534c77c163d6c6dca7
• https://git.kernel.org/stable/c/9dc373f74097edd0e35f3393d6248eda8d1ba99d
• https://git.kernel.org/stable/c/c7f75d11fe72913d2619f97b2334b083cd7bb955
• https://git.kernel.org/stable/c/dc1b438a35773d030be0ee80d9c635c3e558a322
• https://git.kernel.org/stable/c/e407495ba6788a67d1bd41714158c079e340879b
• https://git.kernel.org/stable/c/16d8c44be52e3650917736d45f5904384a9da834
• https://git.kernel.org/stable/c/55fcdd1258faaecca74b91b88cc0921f9edd775d
• https://git.kernel.org/stable/c/6d72e7c767acbbdd44ebc7d89c6690b405b32b57
• https://git.kernel.org/stable/c/8c06f34785068b87e2b560534c77c163d6c6dca7
• https://git.kernel.org/stable/c/9dc373f74097edd0e35f3393d6248eda8d1ba99d
• https://git.kernel.org/stable/c/c7f75d11fe72913d2619f97b2334b083cd7bb955
• https://git.kernel.org/stable/c/dc1b438a35773d030be0ee80d9c635c3e558a322
• https://git.kernel.org/stable/c/e407495ba6788a67d1bd41714158c079e340879b